1. Field of the Invention
The present invention relates to a motor drive device and a motor drive method for use in, for example, electric power steering (EPS) systems that assist in steering of a steering wheel with a motor.
2. Description of the Related Art
In recent years, interest in the functional safety of automobiles has been increasing, and safety when there is sudden loss of assisted control performed by an EPS has been discussed in the Japan Automotive Software Platform and Architecture (JASPAR), Europe, etc. As one of the factors in sudden-loss of assisted control, the influence of voltage fluctuation caused by a decrease in capacity of a power source smoothing capacitor of an inverter is considered. In EPS systems, the mainstream function of current fail-safe (F/S) is to discontinue assistance when there is detection of an abnormality by a current monitor (for example, refer to JP 2004-122943 A).
The decrease in capacity of the capacitor is caused by degradation over the passage of time, temperature changes, etc. When the capacity of the capacitor has decreased, since it is difficult to detect an abnormality by a current monitor in a state in which the amount of assistance is small, the abnormality is detected after the amount of assistance increases. If the assistance is discontinued (manual steering) during traveling of a vehicle in a state in which the amount of assistance is large, there is a concern that a driver may feel a sense of discomfort in steering or steering may suddenly become difficult.
Additionally, in EPS systems, in order to achieve a reduction in size, hybrid capacitors may be adopted instead of electrolytic capacitors. The hybrid capacitors have the advantage in that equivalent series resistance (ESR) is small, tolerance against a ripple current is large, and momentary current evaluation is possible even at low capacity. However, since the capacity is low, the influence of a decrease in capacity becomes greater, and the voltage of an amplifier unit of the inverter fluctuates greatly. As a result, there are problems in that the motor control deviates and steering vibration occurs.